cover ups

Hi there, my name is Sid I have a question regarding tattoo cover ups and thought it would be easiest to ask here I have a black ink design on my upper arm which is quite big ( I would be happy to email you an image) but the original tattooist basically messed up the design big style and I was wondering how easy it is to cover up the existing design with a cover up design? I understand it may be difficult as the black areas take up quite a big part of my arm but can you ink other colours over the black or is the technique to create another design which will blend with the existing black and to use the remaining flesh colour of the arm? please reply to my email address [email removed] thanks